Analyte Sensors and Methods

ABSTRACT

Methods of determining analyte concentration. The methods use a fraction of the predicted total charge, from analyte electrolysis, instead of using time, for determination of a data collection endpoint. The total charge is then extrapolated from the data collection endpoint. The analyte concentration is determined from the total charge.

FIELD OF THE INVENTION

This invention relates to methods for determining the concentration ofan analyte in a sample, and sensors that incorporate those methods.

BACKGROUND OF THE INVENTION

Biosensors, also referred to as analytical sensors or merely sensors,are commonly used to determine the presence and concentration of abiological analyte in a sample. Such biosensors are used, for example,to detect and monitor blood glucose levels in diabetic patients.

The detection and quantification of the analyte level can beaccomplished by, for example, coulometry, amperometry, potentiometry orany combination thereof. For systems using amperometry, the analyteconcentration is generally determined from the average amount of thecurrent, in amps, measured over a predetermined time period. For systemsusing coulometry, the analyte concentration is determined from anintegrated total amount of the charge, in coulombs, measured over theperiod of time for required for substantial completion of sampleelectrolysis. The science of analyte determination is an area of ongoingdevelopment.

SUMMARY OF THE INVENTION

The present disclosure provides methods for the determination of theend-point of sample collection for analyte sensors, and sensorsconfigured to determine an analyte concentration in a sample using thosemethods. The techniques of the present disclosure apply to thosedetermination methods in which the sample, such as in an analyticdevice, is entirely or substantially reacted during the time frame ofthe analysis. An obvious electrochemical example is coulometry, andcertain photometric methods are also analogous.

The techniques of the present disclosure extrapolate the total charge bycontinuously monitoring the measured charge and by continuouslycalculating the extrapolated and total charge, as well as the percentcompletion, as the reaction proceeds toward completion. These techniquesdetermine a data collection endpoint based on a predetermined percentageof electrolysis of analyte, by comparing the measured charge to thetotal charge.

The final measured signal (e.g., for coulometry the signal is charge) istypically the sum of two components, (1) the measured signal or thatsignal which is actually measured prior to the data collection endpoint,and (2) the extrapolated signal, or signal calculated or otherwiseexpected to occur after the data collection endpoint, by the process ofextrapolation. The total signal is the sum of the measured signal andthe extrapolated signal.

The time of the data collection endpoint is the basis for determiningthe relative contributions of the measured and extrapolated signals, aswell as the total signal.

In other words, the data collection endpoint is determined from apercentage of electrolysis of analyte, rather than from a predeterminedtime period or from the fall of the current to a predeterminedpercentage of the initial value.

In some embodiments, the total charge is calculated from extrapolatedcurrent decay from a data collection endpoint in real time, thatendpoint having been determined from a predicted total charge. In someembodiments, the endpoint is at a predetermined percentage of thepredicted total charge. The predicted total charge is used to controlthe current collection process until the point in time that apredetermined fraction of total analyte in the sample is electrolyzed.The method uses a fraction of the predicted total charge instead ofusing current or time for the determination of the data collectionendpoint.

Embodiments of the present invention are used for the detection andquantification of an analyte, for example glucose, from a sample; inmany embodiments the detection and quantification is accomplished with asmall volume, e.g., submicroliter sample. The sensor's sample chambermay be any suitable size, including large and small volume samplechambers. In certain embodiments, such as for small volume samplechambers, the sample chamber is sized to contain no more than about 1 μL(microliter) of sample, in some embodiments no more than about 0.5 μL,in some embodiments no more than about 0.25 μL, and in other embodimentsno more than about 0.1 μL of sample, where in certain embodiments thesample chamber has a volume of no more than about 0.05 μL or even about0.03 μL, or less.

Sensors of the present invention, in some embodiments, may include twosubstrates forming the overall sensor construction, a spacer between thesubstrates, at least one working electrode, at least one counterelectrode, and other optional electrodes. Together, the two substratesand spacer define a sample chamber between the substrates. At least aportion of the working electrode(s) and counter electrode(s) are presentin the sample chamber. The working electrode and counter electrode maybe planar or facing each other.

These and various other features which characterize the invention arepointed out with particularity in the attached claims. For a betterunderstanding of the sensors of the invention, their advantages, theiruse and objectives obtained by their use, reference should be made tothe drawings and to the accompanying description, in which there isillustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, wherein like reference numerals andletters indicate corresponding structure throughout the several views:

FIG. 1 is a graphical example of an analyte measurement, illustratingthe general concepts of measured signal, extrapolated signal, and datacollection endpoint.

FIG. 2 is a graphical comparison of extrapolated current determinedusing a conventional extrapolation technique and extrapolation resultsusing the techniques of the present disclosure.

FIG. 3 is a schematic perspective view of a sensor suitable for use withthe techniques of the present invention.

FIG. 4 is an exploded view of the sensor strip shown in FIG. 3, thelayers illustrated individually.

DETAILED DESCRIPTION

As summarized above, the present disclosure is directed to methods ofcalculating the total charge of an electrolysis reaction, anddetermining an analyte concentration based on that total charge. Thedisclosure is also directed to sensors or biosensors that utilize acalculation for determining the analyte concentration based on afraction of the predicted total charge. “Sensors”, “electrochemicalsensors”, “electrochemical sensor strips”, “biosensors”, and variationsthereof, are devices configured to detect the presence of and/or measurethe concentration of an analyte in a sample via electrochemicaloxidation and reduction reactions. These reactions are transduced to anelectrical signal that can be correlated to an amount or concentrationof analyte. A sensor may be configured as an elongated strip orotherwise.

Various electrochemical sensors, suitable for detection of analyteconcentration in a sample are known. In many embodiments, in use, thesensor is connected to an electrical device, to provide a meter coupledto the sensor. The meter is configured and arranged to determine, duringelectrolysis of a sample in the sample chamber, the total charge,usually from a series of current values. The meter is also configured tocalculate the analyte concentration in the sample based on the totalcharge, total estimated charge or total calculated charge from theelectrolysis of the analyte.

In many embodiments, coulometry is the electroanalytical technique usedfor the current and/or charge determination. Although coulometry has thedisadvantage of requiring the volume of the sample be known, coulometryis a preferred technique for the analysis of small samples (e.g., lessthan 1 microliter) because it has the advantages of, e.g., minimaltemperature dependence for the measurement, minimal enzyme activitydependence for the measurement, minimal redox-mediator activitydependence for the measurement, and no error in the measurement fromdepletion of analyte in the sample.

Coulometry is a method for determining the amount of charge passed orprojected to pass during complete or nearly complete electrolysis of theanalyte. One coulometric technique involves electrolyzing the analyte ona working electrode and measuring the resulting current between theworking electrode and a counter electrode at two or more times duringthe electrolysis. The electrolysis is complete, i.e., 100% electrolyzed,when the current reaches a value near or at zero. The charge used toelectrolyze the sample is then calculated by integrating the measuredcurrents over time and accounting for any background signal. Because thecharge is directly related to the amount of analyte in the sample thereis no temperature dependence of the measurement. In addition, theactivity of the enzyme does not affect the value of the measurement, butonly the time required to obtain the measurement (i.e., less activeenzyme requires a longer time to achieve complete electrolysis of thesample) so that decay of the enzyme over time will not render theanalyte concentration determination inaccurate.

Prior to the invention of this disclosure, in some designs, analyteconcentration has been determined through 100% electrolysis; that is,100% of the analyte has been electrolyzed. Using this technique, thetotal charge measured from the electrolysis is related to the analyteconcentration abiding Faraday's Law. The total charge can be determinedfrom measurements of the electrolysis current, i_(t), over time, t. Aseries of currents (i_(x), i_(x+1), i_(x+2), . . . ) is measured for aseries of times (t_(x), t_(x+1), t_(x+2), . . . ). The current can thenbe integrated over time (e.g., numerically integrated using knownnumerical methods) to give the total charge. The analyte concentrationis then calculated from the total charge. Depending on the sensorconfiguration, 100% electrolysis of the analyte could take up to tens ofseconds or even more.

In order to provide faster results, some designs have the currentmeasurements end after a period of time, e.g., after a predeterminedperiod of time, or after the current has decreased to a predeterminedpercentage of its initial level, when only a fraction or percentage ofthe analyte has been electrolyzed. The subsequent current is calculated,i.e., extrapolated, from the measured current data. The total charge isthen integrated from the measured current points and the extrapolateddata, and analyte concentration is calculated from the total charge.

Data extrapolation is usually based on a simplified mathematic model ofthe expected actual results. However, there are various factors, such asaged or deteriorated sensors, high analyte concentration, highhematocrit percentage, high sample viscosity, etc., that may cause thecurrent decay profile to deviate from expected. As a result, there maybe a significant difference in the extrapolated total charge as comparedto the actual total charge. That is, the extrapolated total charge mayvary substantially, depending on the time of the data collectionendpoint.

The methods of this disclosure utilize values that are directly relatedto the percentage of analyte being electrolyzed, rather than apredetermined time period of electrolysis. In particular, the methodsutilize a percentage of the total charge to select a data collectionendpoint, at which extrapolation begins. In a simplified form, themethods of this disclosure predict the total charge from analyteelectrolysis, take a predetermined percentage of that predicted totalcharge to find a data collection endpoint, and from the current at thatpredetermined data collection endpoint, extrapolate to determine thetotal charge. The extrapolated total charge is then correlated to ananalyte concentration.

In accordance with methods of this disclosure, the predicted totalcharge is used to control the data collecting process, until a time whena predetermined fraction or percentage of analyte in the sample has beenelectrolyzed. The method uses a fraction or percentage of the predictedtotal charge for the determination of the endpoint of data collection,which is directly related to the percentage of analyte being analyzed,instead of merely using time or current, as in previous methods. Fromthe determined data collection endpoint, the total charge isextrapolated, which is then correlated to an analyte concentration.

This calculation technique of the present disclosure is a betterapproach than previous extrapolation techniques, yielding more accurateconcentration values and accommodating wider situations, e.g., hemocritlevels, sample temperature and/or viscosity, etc.

Methods of this disclosure use a predicted total charge to determine thedata collection endpoint. This prediction of total charge is done in‘real time’, during the electrolysis of the analyte. The prediction canbe done by using a regular or conventional data extrapolation algorithm,which generally includes numerous iterative approximation steps, whichmay require the use of a powerful microprocessor in the meter.Alternately, an approximated model can be used for predicting the totalcharge.

A data collection endpoint is selected, based on a predeterminedpercentage of the electrolyzed analyte. The predetermined percentage canbe any percentage greater than 0 (zero) up to 100%. In some embodiments,however, the measured percentage of total charge is at least 40%, and inother embodiments, at least 50%. That is, in some embodiments, at least40% of the analyte has been electrolyzed at the data collectionendpoint, and in other embodiments, at least 50% of the analyte has beenelectrolyzed. The underlying premise of the calculation technique of thepresent disclosure is to use a data collection endpoint that balancesbetween the speed and accuracy of measurement, and which is directlyrelated to the percentage of analyte being electrolyzed. From the datacollection endpoint, the subsequent current is extrapolated. Variousdifferent mathematic models can be used to provide the estimation. Inone embodiment, a linear extrapolation, based on the measured currentdata and the data collection endpoint, is used to calculate a linearlyestimated current and thus a linearly estimated charge, Q_(lin). In someembodiments, only of a portion of the measured current data, usuallyonly that collected, e.g, 1 or 2 seconds prior to the data collectionendpoint, is used for the linear current estimate and the linearlyestimated charge. The linearly estimated charge, Q_(lin), is apercentage, p, of the extrapolated portion of the charge, Q_(e). Thefinal Q_(e)/Q_(total) ratio is calculated by combining with the dynamicmeasured portion of charge Q_(m). as

$\begin{matrix}{\frac{Q_{e}}{Q_{total}} = {\frac{Q_{e}}{Q_{m} + Q_{e}} = \frac{W_{lin}/p}{Q_{m} + {Q_{lin}/p}}}} & (1)\end{matrix}$

Data is collected until Q_(m)=αQ_(lin), where coefficient α controls thethreshold, above which an automatic data correction by collecting moredata will be applied. α can be optimized to balance the measurementspeed and accuracy.

FIG. 1 is a graphical example of an analyte measurement, illustratingthe general concepts of measured signal, extrapolated signal, and datacollection endpoint.

FIG. 2 provides a graphical explanation of the methods of thisdisclosure using one specific example. The solid line, beginning at time0, represents the actual current measurement. At 2 seconds, with theinitial current, I₀, the total current is extrapolated (dashed line) anda total charge is predicted. In this example, the predeterminedelectrolysis percentage is set at 50% of the predicted total charge.Thus, the data collection ends at this predetermined endpoint, which inthis example is approximately 5 seconds. At 5 seconds, a linearextrapolation is made. The total charge is integrated over theextrapolated linear current and the measured current data. From thetotal charge, the analyte concentration is determined.

In most embodiments, the predetermined endpoint for data collection isno more than about 10 seconds, in some embodiments no more than about 5seconds, and in some embodiments, no more than about 3 seconds.

The techniques described above can be used for generally any sensorconfigured for use with coulometry. Referring to FIGS. 3-4, one exampleof an in vitro electrochemical sensor suitable for use with theinvention is schematically illustrated. It is understood that theanalyte measurement procedure of this disclosure could be used with anysensor configuration, and that the sensor strip illustrated in thefigures is representative of only one suitable sensor.

In FIGS. 3-4, an exemplary embodiment of a sensor, suitable for use withthe end-point determination methods of the present disclosure, isschematically illustrated, herein shown in the shape of a sensor strip10. It is to be understood that the sensor may be any suitable shape.Sensor strip 10 has a first substrate 12, a second substrate 14, and aspacer 15 positioned therebetween. Together, these elements define, atleast partially, a sample chamber 20 with an inlet 21 for receiving asample to be analyzed. In some embodiments, the sample chamber is sizedto contain no more than about 1 μL (microliter) of sample, in someembodiments no more than about 0.5 μL, in some embodiments no more thanabout 0.25 μL, and in other embodiments no more than about 0.1 μL ofsample, where in certain embodiments the sample chamber has a volume ofno more than about 0.05 μLor even about 0.03 μL or less.

Sample chamber 20 includes a measurement zone where the sample iselectrolyzed. In some embodiments, the measurement zone is sized tocontain no more than about 1 μL of sample, in some embodiments no morethan about 0.5 μL, in some embodiments no more than about 0.25 μL and inother embodiments no more than about 0.1 μL of sample, where in certainembodiments the measurement zone has a volume of no more than about 0.05μL or even about 0.03 μL or less.

Sensor strip 10 includes at least one working electrode 22 and at leastone counter electrode 24. Sensor strip 10 has a first, distal end 10Aand an opposite, proximal end 10B. At distal end 10A, sample to beanalyzed is applied to sensor 10. Distal end 10A could be referred as‘the fill end’, ‘sample receiving end’, or similar. Inlet 21 ispositioned at or proximate to distal end 10A. Proximal end 10B of sensor10 is configured for operable, and usually releasable, connecting to adevice such as a meter.

Sensor strip 10 is a layered construction, in certain embodiments havinga generally rectangular shape, i.e., its length is longer than itswidth, although other shapes of sensor 10 are possible as well.

Each of the elements of sensor strip 10 is generally well known. Forexample, substrates 12, 14 can be inert substrates (e.g., polymericsubstrates), although other substrates can be used. Electrodes 22, 24and any other electrodes, (e.g., an indicator electrode, an insertionmonitor, etc.) generally comprise a conductive material, such as carbon,silver, gold, platinum, or the like. In the illustrated embodiment,electrodes 22, 24 are facing electrodes, positioned generally oppositeone another on separate substrates 12, 14. Alternate embodiments ofsensors can have electrodes 22, 24 on the same substrate, e.g., asco-planar or planar electrodes.

In some embodiments, sensing chemistry material(s) are provided insample chamber 20 to facilitate the analysis of the analyte. Sensingchemistry material facilitates the transfer of electrons between workingelectrode 22 and the analyte in the sample. Any sensing chemistry may beused in sensor strip 10, and the sensing chemistry may include one ormore materials. The sensing chemistry generally includes an electrontransfer agent that facilitates the transfer of electrons to or from theanalyte. The sensing chemistry may, additionally to or alternatively tothe electron transfer agent, include a redox mediator.

In use, a sample of biological fluid is provided into sample chamber 20of sensor 10, where the level of analyte is determined using the methodsdescribed above. In many embodiments, it is the level of glucose inblood, interstitial fluid, and the like, that is determined. Also inmany embodiments, the source of the biological fluid is a drop of blooddrawn from a patient, e.g., after piercing the patient's skin with alancing device or the like, which may be present in an integrateddevice, together with the sensor strip.

Embodiments of the subject methods include contacting the sensor with afluid sample (obtained, e.g., from a skin incision) and transferring avolume of the fluid to the sample chamber and measurement zone of thesensor. Accordingly, bodily fluid may be first contacted with a portionof one of the substrates of the sensor prior to being contacted with theother substrate and/or sample chamber.

A common use for an analyte sensor of the present invention, such assensor 10, is for the determination of analyte concentration in abiological fluid, such as glucose concentration in blood, interstitialfluid, and the like, in a patient or other user. Additional analytesthat may be determined include but are not limited to, for example,acetyl choline, amylase, bilirubin, cholesterol, chorionic gonadotropin,creatine kinase (e.g., CK-MB), creatine, DNA, fructosamine, glucose,glutamine, growth hormones, hormones, ketones, lactate, peroxide,prostate-specific antigen, prothrombin, RNA, thyroid stimulatinghormone, and troponin. The concentration of drugs, such as, for example,antibiotics (e.g., gentamicin, vancomycin, and the like), digitoxin,digoxin, drugs of abuse, theophylline, and warfarin, may also bedetermined.

Sensor strips 10 may be available at pharmacies, hospitals, clinics,from doctors, and other sources of medical devices. Multiple sensorstrips 10 may be packaged together and sold as a single unit; e.g., apackage of about 25, about 50, or about 100 sensors, or any othersuitable number. A kit may include one or more sensors of the presentinvention, and additional components such as control solutions and/orlancing device and/or meter, etc.

Sensor strips 10 may be used for an electrochemical assay, or, for aphotometric test. Sensor strips 10 are generally configured for use withan electrical meter, which may be connectable to various electronics. Ameter may be available at generally the same locations as sensor strips10, and sometimes may be packaged together with sensor strips 10, e.g.,as a kit.

Examples of suitable electronics connectable to the meter include a dataprocessing terminal, such as a personal computer (PC), a portablecomputer such as a laptop or a handheld device (e.g., personal digitalassistants (PDAs)), and the like. The electronics are configured fordata communication with the receiver via a wired or a wirelessconnection. Additionally, the electronics may further be connected to adata network (not shown) for storing, retrieving and updating datacorresponding to the detected glucose level of the user.

The various devices connected to the meter may wirelessly communicatewith a server device, e.g., using a common standard such as 802.11 orBluetooth RF protocol, or an IrDA infrared protocol. The server devicecould be another portable device, such as a Personal Digital Assistant(PDA) or notebook computer, or a larger device such as a desktopcomputer, appliance, etc. In some embodiments, the server device has adisplay, such as a liquid crystal display (LCD), as well as an inputdevice, such as buttons, a keyboard, mouse or touch-screen. With such anarrangement, the user can control the meter indirectly by interactingwith the user interface(s) of the server device, which in turn interactswith the meter across a wireless link.

The server device may also communicate with another device, such as forsending data from the meter and/or the service device to a data storageor computer. For example, the service device could send and/or receiveinstructions (e.g., an insulin pump protocol) from a health careprovider computer. Examples of such communications include a PDAsynching data with a personal computer (PC), a mobile phonecommunicating over a cellular network with a computer at the other end,or a household appliance communicating with a computer system at aphysician's office.

A lancing device or other mechanism to obtain a sample of biologicalfluid, e.g., blood, from the patient or user may also be available atgenerally the same locations as sensor strips 10 and the meter, andsometimes may be packaged together with sensor strips 10 and/or meter,e.g., as a kit.

Sensor strips 10 are particularly suited for inclusion in an integrateddevice, i.e., a device which has the sensor and a second element, suchas a meter or a lancing device, in the device. The integrated device maybe based on providing an electrochemical assay or a photometric assay.In some embodiments, sensor strips 10 may be integrated with both ameter and a lancing device. Having multiple elements together in onedevice reduces the number of devices needed to obtain an analyte leveland facilitates the sampling process. For example, embodiments mayinclude a housing that includes one or more of the subject strips, askin piercing element and a processor for determining the concentrationof an analyte in a sample applied to the strip. A plurality of strips 10may be retained in a cassette in the housing interior and, uponactuation by a user, a single strip 10 may be dispensed from thecassette so that at least a portion extends out of the housing for use.

EXAMPLES

Blood samples from eight different donors were adjust to five differenthematocrit levels and tested with sensors. The results from the 40%hematocrit samples from the first two donors were used to determinecalibration curves of the strips which were then application to all therest of the data.

The following table compares the results from using the sensor stripswith an “old algorithm” based on end point determined by percent residuecurrent and with the “new algorithm” which is based on a revised datacollection endpoint. For the “old algorithm”, the data collectionendpoint was when the current reading was 50% of the peak reading. Forthe “new algorithm”, the data collection endpoint was when 60% of theanalyte had been electrolyzed. The coefficient α was 3.5.

hematocrit Glucose level Algorithm 15% 25% 40% 55% 65% 50 mg/dl Old 7.494.24 1.19 4.88 5.54 New 3.17 0.21 1.56 2.25 0.40 Δ −4.02 −4.03 0.37−2.62 −5.14 200 mg/dL Old 21.01 13.28 −1.14 −11.60 −11.92 New 19.8012.03 −2.14 −7.57 −5.77 Δ −1.21 −1.25 1.00 −4.03 −6.15 400 mg/dL Old21.89 11.73 −1.10 −16.34 −19.04 new 21.79 11.54 −1.24 −11.94 −8.17 Δ−0.10 −0.19 0.14 −4.40 −10.87

The negative signs represent a reduction in average bias, which isdesirable.

The data shows that at high hematocrit levels, the algorithm improvedthe accuracy, however, there was minimal effect for low hematocritsamples, since the algorithm is focused on errors generated by slowerreactions.

The invention has been described with reference to various specific andpreferred embodiments and techniques. However, it will be apparent toone of ordinarily skill in the art that many variations andmodifications may be made while remaining within the spirit and scope ofthe invention. For example, the invention has described primarily withrespect to an electrochemical sensor strip for exemplary purposes only.It is to be understood that the sensors of the invention may be opticalsensors, etc. and/or those that utilize methods such as amperometry orpotentiometery.

All patents, applications and other references in this specification areindicative of the level of ordinary skill in the art to which thisinvention pertains. All patents, patent applications and otherreferences are herein incorporated by reference to the same extent as ifeach individual patent, application or reference was specifically andindividually incorporated by reference.

1-8. (canceled)
 9. A device for determining the concentration of ananalyte in a sample, the device comprising: a sensor port for receivingan analyte sensor; a display a processor; and a memory comprisinginstructions when executed by the processor cause the device to:electrolyze a sample deposited on the analyte sensor positioned in thesensor port; measure current as a function of time; predict a firstsignal value for complete electrolysis of the analyte from the measuredcurrent prior to a first time during electrolysis and an extrapolatedcurrent subsequent the first time during electrolysis; determine a datacollection endpoint, wherein the data collection endpoint is apredetermined percentage of the predicted first signal value; calculatea second signal value from the measured current prior to the datacollection endpoint and an extrapolated current subsequent the datacollection point; and determine the concentration of the analyte basedon the calculated second signal value.
 10. The device of claim 9,wherein the first signal value is the sum of a measured signal and anextrapolated signal.
 11. The device of claim 10, wherein the signal ischarge, and the first signal value is a predicted total charge forcomplete electrolysis.
 12. The device of claim 11, wherein theelectrolysis comprises coulometry.
 13. The device of claim 11, whereinthe extrapolated current subsequent the data collection point is anextrapolated linear current.
 14. The device of claim 13, whereindetermining the second signal value comprises: integrating theextrapolated linear current and the measured current prior to the datacollection endpoint.
 15. The device of claim 9, wherein electrolysiscomprises amperometry.
 16. The method of claim 13, wherein theextrapolated linear current is calculated based on the measured currentprior to the data collection point and the data collection point. 17.The device of claim 9, wherein the extrapolated linear current iscalculated based on only a portion of the measured current prior to thedata collection point and the data collection point.